| Abstract |
Artificial-tracer tests can provide a useful tool for evaluating kinetic exchange processes in dual-porosity media, if certain requirements are met. Simulated tracer signals (breakthrough curves, BTC) for five typical kinetic-exchange process scenarios (roughly covering the broad range of solute partitioning processes that may become relevant for deep-georeservoir characterization) are seen to respond to variations of kinetic exchange parameter values in a monotonous, and fairly sensitive manner. In (apparent) flow-storage repartition terms (a tool proposed by Shook 2003 for characterizing what Shook deems as reservoir geometry), apparent FSR shapes derived from simulated tracer BTCs are seen, as well, to largely follow the variations in kinetic-exchange parameters. Some of these FSR simulation findings seem somewhat surprising at first sight, but become understandable by comparing the degree of imbalance between fixation and release rates to the degree of deviation from the diagonal shape corresponding to the uniform plug-flow system. BTC simulation findings include: peak height is roughly determined by the fixation rate a, and it decreases with increasing a; the tailing decrease rate is roughly determined by the release rate ß; peak arrival times and peak interval durations (before the onset of late tailings) depend on both a and ß; the tailing decrease rate gets accelerated with increasing ß; the higher the value of ß, the lower the late tailings, and the later the onset of late tailings for a given value of a. For the particular case of matrix diffusion processes in their first-order approximation, the higher the fluid-rock interface area density, the later and lower the peak value, the higher and faster the mid-term tailings, but the lower and longer-lasting the late tailing levels. Apparent FSR findings include: for a given a value, the flow capacity to any given storage capacity decreases with increasing ß (which seems surprising at first sight); in other words, with decreasing fixation-vs-release imbalance (with ß slower than a), the apparent FSR shape approaches the diagonal shape of a uniform plug-flow system; for a given ß value, increasing the value of a will reduce the flow capacity at high storage values, while raising the flow capacity at low storage values. For the particular case of matrix diffusion processes in their first-order approximation, the higher the fluid-rock interface area density, the lower the flow capacity for any given storage capacity, and the closer the apparent FSR shape will approach the diagonal. The latter looks surprising at first sight, but can be understood from the fact that, when a approximately equals ß, both much faster than 1/MRT, the matrix diffusion process approaches an equilibrium-retardation process (towards which FSR shapes are insensitive), while on the other hand the first-order approximation suggested by various authors becomes inadequate for describing matrix diffusion processes. |